Fuel cell with triangular buffers

ABSTRACT

An oxygen-containing gas flow field for supplying an oxygen-containing gas from an oxygen-containing gas supply passage to an oxygen-containing gas discharge passage is formed on a first metal plate. The oxygen-containing gas flow field includes oxygen-containing gas flow grooves as serpentine flow grooves having two turn regions T 1 , T 2 . The oxygen-containing gas flow grooves have substantially the same length. The oxygen-containing gas flow grooves are connected to an inlet buffer and an outlet buffer at opposite ends. The inlet buffer and the outlet buffer have a substantially triangular shape, and are substantially symmetrical with each other.

RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national stage filing ofInternational Application No. PCT/JP2003/13756, filed 28 Oct. 2003,which claims priority to Japanese Patent Application No. 2002-313242filed on 28 Oct. 2002, Japanese Patent Application No. 2002-336742 filed20 Nov. 2002, Japanese Patent Application No. 2002-336753 filed 20 Nov.2002, Japanese Patent Application No. 2003-360900 filed 21 Oct. 2003,and Japanese Patent Application No. 2003-360907 filed 21 Oct. 2003 inJapan. The contents of the aforementioned applications are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a fuel cell formed by alternatelystacking an electrolyte electrode assembly and separators. Theelectrolyte electrode assembly includes a pair of electrodes and anelectrolyte interposed between the electrodes.

2. Background Art

For example, a solid polymer fuel cell employs a polymer ion exchangemembrane as an electrolyte membrane. The electrolyte solid electrolytemembrane is interposed between an anode and a cathode to form a membraneelectrode assembly (electrolyte electrode assembly). Each of the anodeand the cathode is made of electrode catalyst and porous carbon. Themembrane electrode assembly is sandwiched between separators (bipolarplates) to form the fuel cell. In use, generally, a predetermined numberof the fuel cells are stacked together to form a fuel cell stack.

In the fuel cell stack, a fuel gas (reactant gas) such as a gas chieflycontaining hydrogen (hereinafter also referred to as thehydrogen-containing gas) is supplied to the anode. The catalyst of theanode induces a chemical reaction of the fuel gas to split the hydrogenmolecule into hydrogen ions and electrons. The hydrogen ions move towardthe cathode through the electrolyte, and the electrons flow through anexternal circuit to the cathode, creating a DC electric current. Anoxidizing gas (reactant gas) such as a gas chiefly containing oxygen(hereinafter also referred to as the oxygen-containing gas) is suppliedto the cathode. At the cathode, the hydrogen ions from the anode combinewith the electrons and oxygen to produce water.

In the fuel cell, a fuel gas flow field (reactant gas flow field) isformed on a surface of the separator facing the anode for supplying thefuel gas to the anode. An oxygen-containing gas flow field (reactant gasflow field) is formed on a surface of the separator facing the cathodefor supplying the oxygen-containing gas to the cathode. Further, acoolant flow field is provided between adjacent surfaces of theseparators such that a coolant flows along the separators. Generally,fluid supply passages and fluid discharge passages extend through thefuel cell stack in the stacking direction of the separators. The fuelgas flow field, the oxygen-containing gas flow field, and the coolantflow field include plurality of flow grooves extending from the fluidsupply passages to the fluid discharge passages, respectively. The flowgrooves are straight grooves, or serpentine grooves.

However, if openings of the fluid supply passage or the fluid dischargepassage are small for the plurality of flow grooves, it is required toprovide buffers around the fluid supply passage and the dischargepassage, respectively, so that a fluid such as the fuel gas, theoxygen-containing gas, or the coolant can flow along the flow groovessmoothly.

For example, a gas flow field plate of a fuel cell as disclosed inJapanese Laid-Open Patent Publication No. 10-106594 is known. Accordingto the disclosure of the Japanese Laid-Open Patent Publication No.10-106594, as shown in FIG. 12, for example, a gas flow field plate 1for forming a flow field of the oxygen-containing gas includes a groovemember 2 made of carbon or metal. At an upper side of the gas flow fieldplate 1, an inlet manifold 3 for the oxygen-containing gas is provided.At a lower side of the gas flow field plate 1, an outlet manifold 4 forthe oxygen-containing gas is provided.

The groove member 2 has an inlet side channel 5 a connected to the inletmanifold 3, an outlet side channel 5 b connected to the outlet manifold4, and an intermediate channel 6 connected between the inlet sidechannel 5 a and the outlet side channel 5 b. A plurality of protrusions7 a are formed in the inlet side channel 5 a and the outlet side channel5 b such that the inlet side channel 5 a and the outlet side channel 5 bhave matrix patterns. The intermediate channel 6 has a serpentinepattern having a plurality of turn regions. The intermediate channel 6includes a plurality of straight grooves 8 and channels 9 formed at theturn regions. A plurality of protrusions 7 b are formed in the channels9 such that the channels 9 have matrix patterns.

In the gas flow field plate 1 constructed as described above, the inletside channel 5 a and the outlet side channel 5 b function as buffers.Thus, the contact area between the supplied gas and the electrode islarge, and the supplied gas can move freely. Further, in theintermediate channel 6, the reactant gas flows uniformly at high speedthrough the plurality of straight grooves 8.

In the gas flow field plate 1, practically, a plurality of serpentinepassages 1 a extending from the inlet manifold 3 to the outlet manifold4 are formed. In the plurality of the straight grooves 8, the respectivepassages 1 a have substantially the same length. Thus, the flowresistance tends to be constant in each of the passages 1 a.

However, in the inlet side channel 5 a and the outlet side channel 5 bwhich are formed in the matrix patterns by the plurality of protrusions7 a, the passages 1 a from the inlet manifold 3 and the outlet manifold4 to the respective straight grooves 8 have different lengths.Therefore, the flow resistance varies in the inlet side channel 5 a andthe outlet side flow channels 5 b, and thus, it is not possible tosupply the reactant gas uniformly over the entire surface of theelectrode. Consequently, the reactant gas is not distributed desirably.

Likewise, in the matrix pattern channels 9 formed by the plurality ofthe protrusion 7 b, when reactant gas flows out of the respectivestraight grooves 8, turns back in the matrix pattern channels 9, andflows into the respective straight grooves 8, since the flow passages 1a have different lengths, the reactant gas are not distributeduniformly. Thus, the reactant gas is not supplied uniformly over theentire surface of the electrode. Thus, the desired power generationperformance can not be maintained.

Further, a coolant flow field may be formed on the back surface of thegas flow field plate 1 for supplying a coolant along the surface of thegas flow field plate 1. In this case, for example, an inlet manifold 3 aof the coolant is provided adjacent to the inlet manifold 3, and anoutlet manifold 4 b of the coolant is provided adjacent to the outletmanifold 4. The inlet side channel 5 a and the outlet side channel 5 bmay function as buffers for supplying the coolant to the coolant flowfield, and discharging the coolant from the coolant flow field on theback surface of the gas flow field plate 1.

However, the inlet side channel 5 a and the outlet side channel 5 b asthe buffers have a square shape or a rectangular shape. Therefore, theinlet manifolds 3, 3 a, and the outlet manifolds 4, 4 a can not beprovided in a small space on the surfaces of the gas flow field platesefficiently. Therefore, the area of the gas flow field pate 1 which isnot used for reaction increases, and the output density per unit area islowered. Consequently, the gas flow field plate 1 itself has aconsiderably large size.

SUMMARY OF THE INVENTION

The present invention solves this type of problem, and an object of thepresent invention is to provide a fuel cell in which the flow resistancein a reactant gas flow field in a serpentine pattern is uniform, and thereactant gas can be distributed over the entire electrode surfacedesirably, and the desired power generation performance can bemaintained.

Further, another object of the present invention is to provide a fuelcell in which, by specially designing the shape of a buffer, the desiredfunction of the buffer is achieved with relatively small area, theoutput density is improved suitably, and it is possible to downsize thefuel cell easily.

According to the present invention, a reactant gas flow field is formedfor supplying a reactant gas along an electrode surface. The reactantgas flow field includes a plurality of serpentine flow grooves havingsubstantially the same length. The serpentine flow grooves include aneven number of turn regions formed on a surface of the separator. Asubstantially triangular inlet buffer connects the serpentine flowgrooves and a reactant gas supply passage extending through the fuelcell in a stacking direction of the fuel cell. A substantiallytriangular outlet buffer connects the serpentine flow grooves and areactant gas discharge passage extending through the fuel cell in thestacking direction of the fuel cell. The inlet buffer and the outletbuffer are formed substantially symmetrically with each other.

Since the serpentine flow grooves of the reactant gas flow field havesubstantially the same length, the flow resistance is uniform in each ofthe serpentine flow grooves. Further, the entire reactant gas flow fieldfrom the reactant gas supply passage to the reactant gas dischargepassage has the uniform flow resistance, and thus, the reactant gas isdistributed efficiently in the reactant gas flow field. Therefore, thepower generation performance of the fuel cell is maintained effectively.

Further, according to the present invention, a reactant gas flow fieldis formed on one surface of a metal separator for supplying a reactantgas along an electrode surface, and a coolant flow field is formed onthe other surface of the metal separator for supplying a coolant alongthe other surface of the metal separator. The metal separator includes asubstantially triangular buffer. The buffer has one side connected to areactant gas passage on the one surface of the metal separator, andanother side connected to a coolant passage on the other side of themetal separator, and a still another side connected to the reactant gasflow field and the coolant flow field on both surfaces of the metalseparator.

Thus, the buffer has a distribution function of the reactant gas in thereactant gas flow field, a distribution function of coolant in thecoolant flow field. Thus, it is possible to simplify and downsize thebuffer structure. The buffer has a substantially triangular shape. Eachside of the buffer is utilized to maintain the desired area in the flowfield. Thus, in comparison with the buffer having a square orrectangular shape, the desired function is maintained with the smallarea, and the output density per unit area in the entire fuel cell iseffectively improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view showing main components of a fuel cellaccording to an embodiment of the present invention;

FIG. 2 is a cross sectional view showing a part of the fuel cell;

FIG. 3 is a front view showing one surface of a first metal plate;

FIG. 4 is a perspective view showing a coolant flow field formed in aseparator;

FIG. 5 is a front view showing the other surface of the first metalplate;

FIG. 6 is a front view showing one surface of a second metal plate;

FIG. 7 is a front view showing the other surface of the second metalplate;

FIG. 8 is a graph showing the relationship between the position of afuel gas flow field and the flow resistance;

FIG. 9 is a view showing a substantially rectangular inlet buffer;

FIG. 10 is a view showing an inlet buffer having another shape;

FIG. 11 is a view showing an inlet buffer having a still another shape;and

FIG. 12 is a view showing a gas flow field plate of a fuel cell ofJapanese Laid-Open Patent Publication No. 10-106594.

DESCRIPTION OF ILLUSTRATED EMBODIMENT

FIG. 1 is an exploded view showing main components of a fuel cell 10according to an embodiment of the present invention. FIG. 2 is a crosssectional view showing a part of the fuel cell 10.

The fuel cell 10 is formed by stacking a membrane electrode assembly(electrolyte electrode assembly) 12 and separators (metal separators) 13alternately. Each of the separators 13 includes first and second metalplates 14, 16, which are stacked together.

As shown in FIG. 1, at one end of the fuel cell 10 in a directionindicated by an arrow B, an oxygen-containing gas supply passage(reactant gas passage) 20 a for supplying an oxidizing gas (reactantgas) such as an oxygen-containing gas, a coolant supply passage 22 a forsupplying a coolant, and a fuel gas discharge passage (reactant gaspassage) 24 b for discharging a fuel gas (reactant gas) such as ahydrogen-containing gas are arranged vertically in a direction indicatedby an arrow C. The oxygen-containing gas supply passage 20 a, thecoolant supply passage 22 a, and the fuel gas discharge passage 24 bextend through the fuel cell 10 in a stacking direction indicated by anarrow A.

At the other end of the fuel cell 10 in the direction indicated by thearrow B, a fuel gas supply passage (reactant gas passage) 24 a forsupplying the fuel gas, a coolant discharge passage 22 b for dischargingthe coolant, and an oxygen-containing gas discharge passage (reactantgas passage) 20 b for discharging the oxygen-containing gas are arrangedvertically in the direction indicated by the arrow C. The fuel gassupply passage 24 a, the coolant discharge passage 22 b, and theoxygen-containing gas discharge passage 20 b extend through the fuelcell 10 in the direction indicated by the arrow A.

The membrane electrode assembly 12 comprises an anode 28, a cathode 30,and a solid polymer electrolyte membrane 26 interposed between the anode28 and the cathode 30. The solid polymer electrolyte membrane 26 isformed by impregnating a thin membrane of perfluorosulfonic acid withwater, for example.

Each of the anode 28 and cathode 30 has a gas diffusion layer such as acarbon paper, and an electrode catalyst layer of platinum alloysupported on carbon particles. The carbon particles are depositeduniformly on the surface of the gas diffusion layer. The electrodecatalyst layer of the anode 28 and the electrode catalyst layer of thecathode 30 are fixed to both surfaces of the solid polymer electrolytemembrane 26, respectively.

As shown in FIGS. 1 and 3, the first metal plate 14 has anoxygen-containing gas flow field (reactant gas flow field) 32 on itssurface 14 a facing the membrane electrode assembly 12. Theoxygen-containing gas flow field 32 is connected to theoxygen-containing gas supply passage 20 a at one end, and connected tothe oxygen-containing gas discharge passage 20 b at the other end. Asubstantially right triangular (substantially triangular) inlet buffer34 is provided near the oxygen-containing gas supply passage 20 a, and asubstantially right triangular (substantially triangular) outlet buffer36 is provided near the oxygen-containing gas discharge passage 20 b.The inlet buffer 34 and the outlet buffer 36 are formed substantiallysymmetrically with each other. The inlet buffer 34 and the outlet buffer36 include a plurality of bosses 34 a, 36 a, respectively.

The inlet buffer 34 and the outlet buffer 36 are connected by threeoxygen-containing gas flow grooves 38 a, 38 b, 38 c. Theoxygen-containing gas flow grooves 38 a through 38 c extend in parallelwith each other in a serpentine pattern for allowing theoxygen-containing gas to flow back and forth in the direction indicatedby the arrow B, and flows in the direction indicated by the arrow C. Theoxygen-containing gas flow grooves 38 a through 38 c have two turnregions T1, T2, and three straight regions extending in the directionindicated by the arrow B, for example. The oxygen-containing gas flowgrooves 38 a through 38 c have substantially the same length.

As shown in FIG. 3, a vertical section (one side) 34 b of the inletbuffer 34 is oriented toward the direction indicated by the arrow C, andsubstantially perpendicular to a terminal portion of theoxygen-containing gas flow grooves 38 a through 38 c. An oblique section34 c of the inlet buffer 34 faces the oxygen-containing gas supplypassage 20 a. The shape of the oxygen-containing gas supply passage 20 acan be selected from various shapes such as a rectangular shape, aparallelogram shape, or a trapezoidal shape. The inner surface of theoxygen-containing gas supply passage 20 a has an oblique side 37 afacing the inlet buffer 34, and in parallel to the oblique section 34 c.

As described above, the shape of the oxygen-containing gas supplypassage 20 a is selected from various shapes. Further, expansions 39 a,39 b expanding toward the oxygen-containing gas supply passage 20 a maybe provided. The oxygen-containing gas discharge passage 20 b, the fuelgas supply passage 24 a, and the fuel gas discharge passage 24 b havethe structure similar to that of the oxygen-containing gas supplypassage 20 a.

A vertical section (one side) 36 b of the outlet buffer 36 is orientedtoward the direction indicated by the arrow C, and substantiallyperpendicular to a terminal portion of the oxygen-containing gas flowgrooves 38 a through 38 c. The oxygen-containing gas flow grooves 38 athrough 38 c have substantially the same length between the verticalsections 34 b, 36 b. An oblique section 36 c of the outlet buffer 36faces the oxygen-containing gas discharge passage 20 b. The innersurface of the oxygen-containing gas discharge passage 20 b has anoblique side 37 b in parallel to the oblique section 36 c.

A line seal 40 is provided on the surface 14 a of the first metal plate14 around the oxygen-containing gas supply passage 20 a, theoxygen-containing gas discharge passage 20 b, and the oxygen-containinggas flow field 32 for preventing leakage of the oxygen-containing gas.

A surface 14 b of the first metal plate 14 faces a surface 16 a of thesecond metal plate 16 with each other, and a coolant flow field 42 isformed between the surface 14 b of the first metal plate 14 and thesurface 16 a of the second metal plate 16. As shown in FIG. 4, forexample, substantially right triangular (substantially triangular) firstand second inlet buffers 44, 46 and substantially right triangular(substantially triangular) first and second outlet buffers 48, 50 areprovided in the coolant flow field 42. The first and second inletbuffers 44, 46 are provided at opposite sides of the coolant supplypassage 22 a in the direction indicated by the arrow C, and the firstand second outlet buffers 48, 50 are provided at opposite sides of thecoolant discharge passage 22 b in the direction indicated by the arrowC.

The first inlet buffer 44 and the second outlet buffer 50 aresubstantially symmetrical with each other. The second inlet buffer 46and the first outlet buffer 48 are substantially symmetrical with eachother. A plurality of bosses 44 a, 46 a, 48 a, and 50 a, are formed onthe first inlet buffer 44, the second inlet buffer 46, the first outletbuffer 48, and the second outlet buffer 50, respectively.

The coolant supply passage 22 a is connected to the first inlet buffer44 through a first inlet connection passage 52, and connected to thesecond inlet buffer 46 through a second inlet connection passage 54. Thecoolant discharge passage 22 b is connected to the first outlet buffer48 through a first outlet connection passage 56, and connected to thesecond outlet buffer 50 through a second outlet connection passage 58.The first inlet connection passage 52 comprises, for example, two flowgrooves, and the second inlet connection passage 54 comprises, forexample, six flow grooves. Likewise, the first outlet connection passage56 comprises six flow grooves, and the second outlet connection passage58 comprises two flow grooves.

The number of flow grooves in the first inlet connection passage 52 isnot limited to “two”, and the number of flow grooves in the second inletconnection passage 54 is not limited to “six”. Likewise, the number offlow grooves in the first outlet connection passage 56 is not limited to“six”, and the number of flow grooves in the second outlet connectionpassage 58 is not limited to “two”. The number of flow grooves in thefirst inlet connection passage 52 may be the same as the number of flowgrooves in the second inlet connection passage 54, and the number offlow grooves in the first outlet connection passage 56 may be same asthe number of flow grooves in the second outlet connection passage 58.

The first inlet buffer 44 and the first outlet buffer 48 are connectedby straight flow grooves 60, 62, 64, and 66 extending in the directionindicated by the arrow B. The second inlet buffer 46 and the secondoutlet buffer 50 are connected by straight flow grooves 68, 70, 72, and74 extending in the direction indicated by the arrow B. Straight flowgrooves 76, 78 extending in the direction indicated by the arrow B for apredetermined distance are provided between the straight flow groove 66and the straight flow groove 68.

The straight flow grooves 60 through 74 are connected by straight flowgrooves 80, 82 which are extending in the direction indicated by thearrow C. The straight flow grooves 62 through 78 are connected with eachother by straight flow grooves 84, 86 which are extending in thedirection indicated by the arrow C. The straight flow grooves 64, 66,and 76 and the straight flow grooves 68, 70, and 78 are connected witheach other by straight flow grooves 88, 90 which are extending in thedirection indicated by the arrow C, respectively.

The coolant flow field 42 is partially defined by grooves on the surface14 b of the first metal plate 14, and partially defined by grooves onthe surface 16 a of the second metal plate 16. The coolant flow field 42is formed between the first metal plate 14 and the second metal plate 16when the first metal plate 14 and the second metal plate 16 are stackedtogether. As shown in FIG. 5, the grooves of the coolant flow field 42is partially formed on the surface 14 b where the grooves of theoxygen-containing gas flow field 32 are not formed on the surface 14 a.

Protrusions on the surface 14 b formed by the grooves of theoxygen-containing gas flow field 32 on the surface 14 a are not shownfor ease of understanding. Likewise, in FIG. 6, protrusions on thesurface 16 b formed by the grooves of the fuel gas flow field (reactantgas flow field) 96 on the surface 16 a are not shown.

The first inlet buffer 44 connected to the coolant supply passage 22 athrough the first inlet connection passage 52 comprising the two flowgrooves is provided on the surface 14 b. Further, the outlet buffer 50connected to the coolant discharge passage 22 b through the secondoutlet connection passage 58 comprising the two flow grooves is providedon the surface 14 b.

Grooves 60 a, 62 a, 64 a, and 66 a connected to the first inlet buffer44 extend discontinuously in the direction indicated by the arrow B fora predetermined distance at intervals. The grooves 60 a, 62 a, 64 a, and66 a are formed where the turn region T2 of the oxygen-containing gasflow grooves 38 a through 38 c and the outlet buffer 36 are not formed.Grooves 68 a, 70 a, 72 a, and 74 a connected to the second outlet buffer50 extend in the direction indicated by the arrow B. The grooves 68 a,70 a, 72 a, and 74 a are formed where the turn region T1 of theoxygen-containing gas flow grooves 38 a through 38 c and the inletbuffer 34 are not formed.

The grooves 60 a through 78 a are part of the straight flow grooves 60through 78, respectively. Grooves 80 a through 90 a of the straight flowgrooves 80 through 90 extend in the direction indicated by the arrow Cfor a predetermined distance where the serpentine oxygen-containing gasflow grooves 38 a through 38 c are not formed.

As shown in FIG. 6, the grooves of the coolant flow field 42 ispartially formed on the surface 16 a of the second metal plate 16 wherethe grooves of the fuel gas flow field 96 as described later are notformed. Specifically, the second inlet buffer 46 connected to thecoolant supply passage 22 a, and the first outlet buffer 48 connected tothe coolant discharge passage 22 b are provided.

Grooves 68 b through 74 b of the straight flow grooves 68 through 74connected to the second inlet buffer 46 extend discontinuously in thedirection indicated by the arrow B for a predetermined distance atintervals. Grooves 60 b through 66 b of the straight flow grooves 60through 66 connected to the first outlet buffer 48 extend in apredetermined pattern. On the surface 16 a, grooves 80 b through 90 b ofthe straight flow grooves 80 through 90 extend in the directionindicated by the arrow C.

In the coolant flow field 42, at part of the straight flow grooves 60through 78 extending in the direction indicated by the arrow B, thegrooves 60 a through 78 a and the grooves 60 b through 78 b face eachother to form a main flow field. The sectional area of the main flowfield in the coolant flow field 42 is twice as large as the sectionalarea of the other part of the coolant flow field 42 (see FIG. 4). Thestraight flow grooves 80 through 90 are partially defined by grooves onboth surfaces 14 b, 16 a of the first and second metal plate 14, 16,partially defined on one surface 14 b of the first metal plate 14, andpartially defined on one surface 16 a of the second metal plate 16. Aline seal 40 a is formed around the coolant flow field 42 between thesurface 14 b of the first metal plate 14 and the surface 16 a of thesecond metal plate 16.

As shown in FIG. 1, when the first and second metal plates 14, 16 arestacked together, the inlet buffer 34 and the second inlet buffer 46 areat least partially overlapped with each other, and the outlet buffer 36and the first outlet buffer 48 are at least partially overlapped witheach other. As shown in FIG. 3, on the surface (one surface) 14 a of thefirst metal plate 14, the inlet buffer 34 includes the oblique section34 c as one side connected the oxygen-containing gas supply passage 20a, and a short side section 34 d as another side, a vertical section 34b as still another side connected to the oxygen-containing gas flowfield 32.

On the surface (the other surface) 16 a of the second metal plate 16, asshown in FIG. 6, the second inlet buffer 46 includes the oblique section46 c as one side, a short side section 46 d as another side, and avertical section 46 b as still another side. On the surface 16 a, theshort side section 46 d of the second inlet buffer 46 is connected tothe coolant supply passage 22 a, and the vertical section 46 b of theinlet buffer 46 is connected to the coolant flow field 42.

As shown in FIG. 3, on the surface 14 a of the first metal plate 14, theoutlet buffer 36 includes the oblique section 36 c as one side connectedto the oxygen-containing gas discharge passage 20 b, and a short sidesection 36 d as another side, and a vertical section 36 b as stillanother side connected to the oxygen-containing gas flow field 32.

As shown in FIG. 6, on the surface 16 a of the second metal plate 16,the first outlet buffer 48 includes an oblique section 48 c as one side,a short side section 48 d as another side connected to the coolantdischarge passage 22 b, and a vertical section 48 b as still anotherside connected to the coolant flow field 42.

As shown in FIG. 7, the second metal plate 16 has the fuel gas flowfield 96 on its surface 16 b facing the membrane electrode assembly 12.The fuel gas flow field 96 includes a substantially right triangular(substantially triangular) inlet buffer 98 provided near the fuel gassupply passage 24 a, and a substantially right triangular (substantiallytriangular) outlet buffer 100 provided near the fuel gas dischargepassage 24 b.

The inlet buffer 98 and the outlet buffer 100 are formed substantiallysymmetrically with each other. The inlet buffer 98 and the outlet buffer100 include a plurality of bosses 98 a, 100 a, respectively. Forexample, the inlet buffer 98 and the outlet buffer 100 are connected bythree fuel gas flow grooves 102 a, 102 b, 102 c. The fuel gas flowgrooves 102 a through 102 c extend in parallel with each other in aserpentine pattern for allowing the fuel gas to flow back and forth inthe direction indicated by the arrow B, and flows in the directionindicated by the arrow C. The fuel gas flow grooves 38 a through 38 care substantially serpentine flow grooves having two turn regions T3,T4, and three straight regions, for example. The fuel gas flow grooves38 a through 38 c have substantially the same length.

A vertical section (one side) 98 b of the inlet buffer 98 is orientedtoward the direction indicated by the arrow C, and substantiallyperpendicular to a terminal portion of the fuel gas flow grooves 102 athrough 102 c. An oblique section 98 c of the inlet buffer 98 faces thefuel gas supply passage 24 a. The inner surface of the fuel gas supplypassage 24 a has an oblique side 104 a facing the oblique section 98 c,and in parallel to the oblique section 98 c. A vertical section (oneside) 100 b of the outlet buffer 100 is oriented toward the directionindicated by the arrow C, and substantially perpendicular to a terminalportion of the fuel gas flow grooves 102 a through 102 c. An obliquesection 100 c of the outlet buffer 100 faces the fuel gas dischargepassage 24 b. The inner surface of the fuel gas discharge passage 24 bhas an oblique side 104 b in parallel to the oblique section 100 c. Onthe surface 16 b, a line seal 40 b is provided around the fuel gas flowfield 96.

As shown in FIGS. 5 and 7, the first inlet buffer 44 formed on thesurface (one surface) 14 b of the first metal plate 14 and the outletbuffer 100 formed on the surface (the other surface) 16 b of the secondmetal plate 16 are overlapped with each other, and the second outletbuffer 50 on the surface 14 b and the inlet buffer 98 on the surface 16b are overlapped with each other.

The first inlet buffer 44 has the oblique section 44 c as one side, theshort side section 44 d as another side, and the vertical section 44 bas still another side. The outlet buffer 100 has the oblique section 100c as one side, a short side section 100 d as another side, and avertical section 100 d as still another side. Likewise, the secondoutlet buffer 50 has an oblique section 50 c as one side, a short sidesection 50 d as another side, and a vertical section 50 b as stillanother side. The inlet buffer 98 has the oblique section 98 c as oneside, a short side section 98 d as another side, and the verticalsection 98 b as still another side.

Next, operation of the fuel cell 10 according to the first embodimentwill be described.

As shown in FIG. 1, an oxidizing gas such as an oxygen-containing gas issupplied to the oxygen-containing gas supply passage 20 a, a fuel gassuch as a hydrogen-containing gas is supplied to the fuel gas supplypassage 24 a, and a coolant such as pure water, an ethylene glycol or anoil are supplied to the coolant supply passage 22 a.

The oxygen-containing gas flows from the oxygen-containing gas supplypassage 20 a into the oxygen-containing gas flow field 32 of the firstmetal plate 14. As shown in FIG. 3, the oxygen-containing gas flowsthrough the inlet buffer 34, and is distributed into theoxygen-containing gas flow grooves 38 a through 38 c. Theoxygen-containing gas flows through the oxygen-containing gas flowgrooves 38 a through 38 c in a serpentine pattern along the cathode 30of the membrane electrode assembly 12 to induce a chemical reaction atthe cathode 30.

The fuel gas flows from the fuel gas supply passage 24 a into the fuelgas flow field 96 of the second metal plate 16. As shown in FIG. 7, thefuel gas flows through the inlet buffer 98, and is distributed into thefuel gas flow grooves 102 a through 102 c. The fuel gas flows throughthe fuel gas flow grooves 102 a through 102 c in a serpentine patternalong the anode 28 of the membrane electrode assembly 12 to induce achemical reaction at the anode 28.

In the membrane electrode assembly 12, the oxygen-containing gassupplied to the cathode 30, and the fuel gas supplied to the anode 28are consumed in the electrochemical reactions at catalyst layers of thecathode 30 and the anode 28 for generating electricity.

After the oxygen-containing gas is consumed at the cathode 30, theoxygen-containing gas flows into the oxygen-containing gas dischargepassage 20 b through the outlet buffer 36. Likewise, after the fuel gasis consumed at the anode 28, the fuel gas flows into the fuel gasdischarge passage 24 b through the outlet buffer 100.

The coolant supplied to the coolant supply passages 22 a flows into thecoolant flow field 42 between the first and second metal plates 14, 16.As shown in FIG. 4, the coolant from the coolant supply passage 22 aflows through the first and second inlet connection passages 52, 54 inthe direction indicated by the arrow C, and flows into the first andsecond inlet buffers 44, 46.

The coolant is distributed from the first and second inlet buffers 44,46 into the straight flow grooves 60 through 66, and 68 through 74, andflows horizontally in the direction indicated by the arrow B. Thecoolant also flows through the straight flow grooves 80 through 90, 76,and 78. Thus, the coolant is supplied to the entire power generationsurface of the membrane electrode assembly 12. Then, the coolant flowsthrough the first and second outlet buffers 48, 50, and flows into thecoolant discharge passages 22 b through the first and second outletconnection passages 56, 58.

In the present embodiment, as shown in FIG. 7, the fuel gas flow field96 includes three fuel gas flow grooves 102 a through 102 c having thetwo turn regions T3, T4 on the surface 16 b. The fuel gas flow grooves102 a through 102 c have substantially the same length. Therefore, theflow resistance in the fuel gas flow grooves 102 a through 102 c isuniform. Thus, the fuel gas is supplied along the fuel gas flow grooves102 a through 102 c uniformly.

Further, the fuel gas flow field 96 has the inlet buffer 98 and theoutlet buffer 100 each having a substantially triangular shape. Theinlet buffer 98 and the outlet buffer 100 are formed substantiallysymmetrically with each other. Thus, as shown in FIG. 8, at both ends ofthe fuel gas flow grooves 102 a through 102 c, the sums of the flowresistance in the inlet buffer 98 and the flow resistance in the outletbuffer 100 are substantially the same.

Thus, the flow resistance is uniform in the entire fuel gas flow field96 from the fuel gas supply passage 24 a to the fuel gas dischargepassage 24 b. The fuel gas is distributed desirably in the fuel gas flowfield 96. Therefore, the fuel gas is supplied to the entire electrodesurface of the anode 28 uniformly and reliably.

Since the plurality of bosses 98 a, 100 a are provided in the inletbuffer 98 and the outlet buffer 100, the fuel gas is distributeduniformly, and the mechanical strength is improved for reliablysupporting the adjacent membrane electrode assembly 12.

Further, since each of the inlet buffer 98 and the outlet buffer 100 hasa substantially triangular shape, the area of the inlet buffer 98 andthe area of the outlet buffer 100 are small in comparison withconventional rectangular buffers. Thus, the space needed for the inletbuffer 98 and the outlet buffer 100 is reduced significantly, and it iseasy to downsize the separator 13 itself.

Further, the oblique section 98 c of the inlet buffer 98 faces, and isin parallel to the oblique side 104 a of the fuel gas supply passage 24a. The oblique section 10 c of the outlet buffer 100 faces, and is inparallel to the oblique side 104 b of the fuel gas discharge passage 24b. Thus, with the simple structure, each of the fuel gas supply passage24 a and the fuel gas discharge passage 24 b has the desired crosssectional area.

Further, the vertical section 98 b of the inlet buffer 98 and thevertical section 100 b of the outlet buffer 100 are substantiallyperpendicular to the terminal portions of the fuel gas flow grooves 102a through 102 c. Thus, the fuel gas smoothly flows from the inlet buffer98 into the fuel gas flow grooves 102 a through 102 c, and flows out ofthe fuel gas flow grooves 102 a through 102 c to the outlet buffer 100.

As shown in FIG. 3, as with the fuel gas flow field 96, in theoxygen-containing gas flow field 32, the three oxygen-containing gasflow grooves 38 a through 38 c are serpentine flow grooves havingsubstantially the same length. The inlet buffer 34 and the outlet buffer36 provided at opposite ends of the oxygen-containing gas flow grooves38 a through 38 c have a substantially triangle shape, and aresymmetrical with each other.

Thus, it is possible to ensure that the flow resistance is uniform inthe entire oxygen-containing gas flow field 32 from theoxygen-containing gas supply passage 20 a to the oxygen-containing gasdischarge passage 20 b. The oxygen-containing gas is distributedefficiently in the oxygen-containing gas flow field 32. Therefore, it ispossible to supply the oxygen-containing gas over the entire electrodesurface of the cathode 30. Accordingly, the power generation performanceof the fuel cell 10 is maintained effectively.

In the present embodiment, as shown in FIG. 1, when the first and secondmetal plates 14, 16 are stacked together, the inlet buffer 34 and thesecond inlet buffer 46 are overlapped in the stacking direction. Theinlet buffer 34 and the second inlet buffer 46 have a substantiallytriangular shape (substantially right triangular shape). As shown inFIG. 3, on the surface 14 a of the first metal plate 14 (one surface ofthe separator 13), the oblique section 34 c of the inlet buffer 34 isconnected to the oxygen-containing gas supply passage 20 a, and thevertical section 34 b of the inlet buffer 34 is connected to theoxygen-containing gas flow field 32.

Further, as shown in FIG. 6, on the surface 16 a of the second metalplate 16 (the other surface of the separator 13), the short side section46 d of the second inlet buffer 46 is connected to the coolant supplypassage 22 a, and the vertical section 46 b of the second inlet buffer46 is connected to the coolant flow field 42.

Thus, in the separator 13, the inlet buffer 34 and the second inletbuffer 46 are overlapped together to form a single buffer. The bufferhas the function of distributing the oxygen-containing gas in theoxygen-containing gas flow field 32, and the function of distributingthe coolant in the coolant flow field 42. Thus, it is possible tosimplify and downsize the structure of the buffer.

The inlet buffer 34 and the second inlet buffer 46 have a substantiallytriangular shape. Each side of the buffers is utilized to achieve thedesired sectional area of the flow field.

Thus, for example, as shown in FIG. 9, in comparison with the case wherea substantially rectangular inlet buffer 110 is provided, and theoxygen-containing gas supply passage 112 having the opening sectionalarea equal to that of the oxygen-containing gas supply passage 20 a, thewidth of the first metal plate 14 is reduced by a distance H.

Accordingly, the inlet buffer 34 can maintain the desirable functionwith the smaller area in comparison with the inlet buffer 110. Thus, inthe present embodiment, it is possible to efficiently improve the outputdensity per unit area in the entire fuel cell 10.

The oblique sections 34 c, 46 c of the inlet buffer 34 and the secondinlet buffer 46 face the oblique side 37 a of the oxygen-containing gassupply passage 20 a, and are in parallel with the oblique side 37 a.Thus, with the compact structure, the desired opening cross sectionalarea of the oxygen-containing gas supply passage 20 a is achieved.

Further, the vertical section 34 b of the inlet buffer 34 and thevertical section 36 b of the outlet buffer 36 are perpendicular to theterminal portions of the oxygen-containing gas flow field grooves 38 athrough 38 c. Therefore, the oxygen-containing gas smoothly flows fromthe inlet buffer 34 into the oxygen-containing gas flow grooves 38 a to38 c, and smoothly flows out of the oxygen-containing gas flow grooves38 a to 38 c to the outlet buffer 36.

Further, as shown in FIG. 1, the outlet buffer 36 and the first outletbuffer 48 are overlapped with each other to form the single bufferhaving a substantially triangular shape. The outlet buffer 36 and thefirst outlet buffer 48 achieve the same advantages as with the inletbuffer 34 and the second inlet buffer 46. Further, the first inletbuffer 44 and the outlet buffer 100 are overlapped with each other, andthe second outlet buffer 50 and the inlet buffer 98 are overlapped witheach other to achieve the same advantages as with the inlet buffer 34and the second inlet buffer 46.

Further, in the present embodiment, the first and second metal plates14, 16 are stacked together to form the separator 13. Therefore, withthe simple structure, the oxygen-containing gas flow field 32, the fuelgas flow field 96, and the coolant flow field 42 are formed easily inthe desired pattern such as a serpentine pattern, and the overall sizeof the fuel cell 10 is reduced. Further, since the serpentine flowpassages are formed, the length of the flow passages is long, and thepressure loss is generated to improve the flow speed.

Further, the oxygen-containing gas flow grooves 38 a through 38 c andthe fuel gas flow grooves 102 a through 102 c are formed in a serpentinepattern having the two turn regions and three straight regions. However,the present invention is not limited in this respect. Any even number ofturn regions, such as four turn regions or six turn regions may beprovided.

Further, in the present embodiment, the inlet buffer 34 has beendescribed as the substantially triangular buffer. However, the presentinvention is not limited in this respect. FIG. 10 shows a buffer 120having a substantially rectangular shape (including a substantiallytrapezoidal shape) including a bottom section 120 a and an upper sidesection 120 b. FIG. 11 shows a buffer 130 having a substantiallytriangular shape including an oblique bottom section 130 a.

1. A fuel cell formed by stacking an electrolyte electrode assembly andseparators alternately, said electrolyte electrode assembly including apair of electrodes and an electrolyte interposed between saidelectrodes, wherein a reactant gas supply passage and a reactant gasdischarge passage extend through said fuel cell in a stacking directionof said fuel cell; a reactant gas flow field is formed for supplying areactant gas along an electrode surface; said reactant gas flow fieldincludes a plurality of serpentine flow grooves having substantially thesame length, said serpentine flow grooves including an even number ofturn regions formed on a surface of said separator; a substantiallytriangular inlet buffer for connecting said serpentine flow grooves andsaid reactant gas supply passage; a substantially triangular outletbuffer for connecting said serpentine flow grooves and said reactant gasdischarge passage; wherein the reactant gas supply passage and thereactant gas discharge passage are positioned on extensions of therespective terminal portions of the serpentine flow grooves, each of theinlet buffer and the outlet buffer has a first side that forms aninterface with terminal portions of the serpentine flow grooves, thefirst side of each of said inlet buffer and said outlet buffer issubstantially perpendicular to the terminal portions of said serpentineflow grooves; each of the inlet buffer and the outlet buffer has asecond side that is substantially parallel to the terminal portions ofsaid serpentine flow grooves; each of the inlet buffer and the outletbuffer has a third side that is oblique to the first and second sides ofeach of the inlet buffer and the outlet buffer, the third oblique sideof each of the inlet buffer and the outlet buffer is substantiallyparallel to one side of each of the reactant gas supply passage and thereactant gas discharge passage; and said inlet buffer and said outletbuffer are formed substantially symmetrically with each other.
 2. A fuelcell according to claim 1, wherein a plurality of bosses are formed inat least one of said inlet buffer and said outlet buffer.
 3. A fuel cellaccording to claim 1, wherein each of said reactant gas supply passageand said reactant gas discharge passage has at least one oblique side;and said oblique side of said reactant gas supply passage faces theoblique side of said inlet buffer, and said oblique side of saidreactant gas discharge passage faces the oblique side of said outletbuffer.
 4. A fuel cell according to claim 1, wherein said fuel cellincludes a coolant supply passage and a coolant discharge passage, saidreactant gas supply passage includes a fuel gas supply passage and anoxygen-containing gas supply passage, and said reactant gas dischargepassage includes a fuel gas discharge passage and an oxygen-containinggas discharge passage; and among six passages comprising said fuel gassupply passage, said oxygen-containing gas supply passage, said coolantsupply passage, said fuel gas discharge passage, said oxygen-containinggas discharge passage, and the coolant discharge passage, three passagesextend through a left end of said separator, and the other threepassages extend through a right end of said separator.